Second Marriage of a Wealthy Old Man - Chapter 21

Leo is an academic genius who enrolled in one of the top universities in the world for the rich and powerful, but he was born in a common family, which was more than enough of a reason for the other students to mess with him.

In the current era, people are either normal or are with ‘special abilities’. Cases related to the latter are managed by the “National Anomaly Prevention and Control and Special Species Management Office”.
Qi Zheng Emperor Sheng Lingyuan was reawakened a thousand years later after he jumped off the fiery Red Lake. Xuan Ji, a newly transferred staff to the Reparations Division of the Deviant Control Office, finds himself having to clean up the mess of a revived tyrannical monarch. The two seemed to share something from the past, but unbeknownst to them, a big conspiracy is stirring right underneath their noses that could destroy the balance of the world.

- Description from Novelupdates

This legend tells the tale of the protagonist, Kuroki summoned to protect the demon king from the hero.

Kuroki, unbeknownst to him, suddenly came to another world. He was summoned by the demon king, Modes. The territory which the demon king overrules was in the midst of being destroyed because of the hero. He had no choice but to bow before Kuroki.

“Please save us, Savior Dono!”

Kuroki nodded, deciding to save the demon king due to an unexpected turn of event.

Gu Changge, a noble lady from a wealthy family, is a well-known business legend in Yuncheng, going into business at 15, taking over as President of Gu Group at 18 and becoming Chairman of the family business at 30.
She is murdered by her husband Shao Tianze, just to give his mistress Gu Changle a healthy heart. But a premeditated car accident makes Gu Changge revive in a new guise and settle into an 18-year-old girl, Song Yunxuan.
The new face, young body and experienced soul become her weapons of revenge. In her struggle with the four big families in Yuncheng, can Gu Changge shrink back? Will she open her broken heart again? Can she always head off dangers in power struggles?
Let’s find out if Gu Changge can revenge herself and create glory again.

- Description from Novelupdates